Nd-Fe-B PERMANENT MAGNETIC MATERIAL AND PREPARATION METHOD THEREOF

ABSTRACT

The present disclosure discloses a permanent magnetic material comprising an Nd—Fe—B alloy and an additive including at least a cobalt ferrite, and a method for preparing a permanent magnetic material. The method comprises steps of mixing an Nd—Fe—B alloy and an additive including at least a cobalt ferrite to obtain a mixture; magnetically orienting and pressing the mixture in a magnetic filed; and sintering and tempering the mixture under the protection of vacuum or an inert gas.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims is a §371 national stage patentapplication based on international Patent Application No.PCT/CN2010/072854, filed on May 17, 2010, entitled “Nd—Fe—B PermanentMagnetic Material and Preparation Method Thereof,” which claims thepriority and benefit of Chinese Patent Application No. 200910107649.2filed with the State Intellectual Property Office of P.R. China on May27, 2009, the entirety of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure relates to an Nd—Fe—B permanent magnetic materialand a preparation method thereof

BACKGROUND

Because of their magnetic properties, low cost and ample reserves,Nd—Fe—B permanent magnetic materials are widely used in vehicles,computers, electronics, mechanical and medical devices, etc. Inaddition, because of their high performance/price ratio, Nd—Fe—Bmaterials have been the ideal materials to produce magnetic devices withhigh efficiency, small volume and light mass. However, as the continuousexpansion of application fields and the development of technology,requirements for performance, operating temperature and corrosionresistance of permanent magnetic materials become higher and higher.

SUMMARY OF THE INVENTION

In view thereof, the present disclosure is directed to provide anNd—Fe—B permanent magnetic material with good high temperature andcorrosion resistance properties, and further to provide a preparationmethod of the Nd—Fe—B permanent magnetic material.

An embodiment of a first aspect of this disclosure provides a permanentmagnetic material with good high temperature and corrosion resistanceproperties, comprising an Nd—Fe—B alloy and an additive comprising acobalt ferrite.

An embodiment of a second aspect of this disclosure provides a method ofpreparing the permanent magnetic material described above, comprisingsteps of mixing an Nd—Fe—B alloy and an additive including at least acobalt ferrite to obtain a mixture; magnetically orienting and pressingthe mixture in a magnetic filed; and sintering and tempering the mixtureunder protection of vacuum or an inert gas.

According to an embodiment of the present disclosure, the cobalt ferritemay be about 0.5 wt % to about 10 wt % of the Nd—Fe—B alloy.

According to an embodiment of the present disclosure, an averageparticle diameter of the cobalt ferrite may range of about 10 nanometersto about 150 nanometers.

According to an embodiment of the present disclosure, the cobalt ferritemay be represented by a general formula of Co_(n)Fe_(3−n)O₄, where n maybe greater than about 0.1 and less then about 2.0.

According to an embodiment of the present disclosure, the Nd—Fe—B alloymay be represented by a general formula ofNd_(a)Re_(b)Fe_((100-a-b-c-d))B_(c)M_(d), where: Re is at least oneelement selected from the group consisting of Pr, Dy, Tb, Ho, Gd, La, Ceand Y; M is at least one element selected from the group consisting ofCo, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d are atomic weightratios, in which a is in a range of about 1≦a≦10, b is in a range ofabout 5≦b≦12, c is in a range of about 5≦c≦8, and d is in a range ofabout 0≦d≦15.

Additional aspects and advantages of the embodiments of presentdisclosure will be given in part in the following descriptions, becomeapparent in part from the following descriptions, or be learned from thepractice of the embodiments of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

These and other aspects, solutions and advantages of the disclosure willbecome apparent and more readily appreciated from the followingdescriptions.

In a first aspect of the present disclosure, an embodiment of thepresent disclosure provides a permanent magnetic material, which maycomprise an Nd—Fe—B (neodymium-iron-boron alloy and an additiveincluding at least a cobalt ferrite. The inventors of the presentdisclosure have been found: by adding particles of a cobalt ferrite anddistributing them uniformly along the grain boundary of the Nd—Fe—Balloy, the over-growth of the grain and magnetic domain size of theNd—Fe—B alloy may be inhibited (i.e. pinning effect), thus improving theoperating temperature effectively, and the cobalt element itself andneodymium can produce stable intergranular additional structure, thusimproving the corrosion resistance property. The content of the heavymetal cobalt may be reduced because of adding a nano-cobalt ferrite,thus lowering the cost. An appropriate amount of oxygen in the cobaltferrite may improve the high temperature resistance properties of thepermanent magnetic material. Meanwhile, due to the presence of thecobalt ferrite, the corrosion resistance property of the permanentmagnetic material may be improved greatly.

In some embodiments, the cobalt ferrite may be about 0.1 wt % to about20 wt %, particularly about 0.5 wt % to about 10 wt % of the Nd—Fe—Balloy. In some embodiments, the average particle diameter of the cobaltferrite is about 20 nanometers to about 60 nanometers. In someembodiments, the cobalt ferrite is represented by a general formula ofCo_(n)Fe_(3−n),O₄, in which n is in a range of about 0.1≦n≦2.0. Theparticles of the cobalt ferrite are distributed uniformly along thegrain boundary of the main phase of the Nd—Fe—B alloy, thus forming thepinning effect. However, the content of cobalt may not exceed about 20wt % of the total weight of the Nd—Fe—B permanent magnetic material,otherwise the coercive force may be seriously reduced.

In some embodiments, the Nd—Fe—B alloy is represented by a generalformula of Nd_(a)Re_(b)Fe_((100-a-b-c-d))B_(c)M_(d), where Re is atleast one element selected from the group consisting of Pr, Dy, Tb, Ho,Gd, La, Ce and Y; M is at least one element selected from the groupconsisting of Co, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d areatomic weight ratios, in which a is in a range of about 1≦a≦10, b is ina range of about 5≦b≦12, c is in a range of about 5≦c≦8, and d is in arange of about 0≦d≦15.

In a second aspect of the present disclosure, an embodiment of thepresent disclosure provides a method of preparing a permanent magneticmaterial, comprising steps of mixing an Nd—Fe—B alloy and an additiveincluding at least a cobalt ferrite to obtain a mixture; magneticallyorienting and pressing the mixture in a magnetic filed; and sinteringand tempering the mixture under protection of vacuum or an inert gas. Insome embodiments, the sintering and tempering can be performed under theprotection of vacuum. In some embodiments, the sintering and temperingstep can be performed under the protection of an inert gas. In someembodiments, the method of preparing a permanent magnetic materialemploying the sintering process may include without limitation at leastone of the following steps: formulating, melting, crushing, milling,magnetically orienting and pressing in a magnetic field, sintering invacuum, mechanical processing and electroplating.

Some of the steps of the method are described as follows:

(1) The Nd—Fe—B alloy may be crushed and milled to form a powder. Thecrushing process may be a hydrogen decrepitation process or a mechanicalcrushing process using a crusher. In some embodiments, jet milling andball milling under an inert gas may be utilized to produce a powder withan average particle diameter of about 2 microns to about 10 microns. Insome embodiments, the Nd—Fe—B alloy may be an Nd—Fe—B alloy ingot or astrip casting flake, both of which are commercially available. TheNd—Fe—B alloy ingot can be prepared by a casting process, and the stripcasting flake can be prepared by a strip casting flaking process. Insome embodiments, the Nd—Fe—B alloy may be represented by the followinggeneral formula: Nd_(a)Re_(b),Fe_((100-a-b-c-d))B_(c)M_(d) where Re isat least one element selected from the group consisting of Pr, Dy, Tb,Ho, Gd, La, Ce and Y; M is at least one element selected from the groupconsisting of Co, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d areatomic weight ratios, in which a is in a range of about 1≦a≦10, b is ina range of about 5≦b≦12, c is in a range of about 5≦c≦8, and d is in arange of about 0≦d≦15.

The casting process may be those well known in the art, and may comprisesteps of casting a melted alloy melt in a water-cooled copper mould. TheNd—Fe—B alloy ingot may comprise columnar crystals, where the columnarcrystals are separated by Nd-rich phase thin layers. Particularly, thedistance between two adjacent Nd-rich phase layers may be about 100microns to about 1500 microns.

In some embodiments, the strip casting flaking process may be those wellknown in the art, and may comprise steps of pouring a melted alloy ontoa rotating copper roller surface, with a rotating linear velocity of thecopper roller surface ranging from about 1 meter per second (“m/s”) toabout 2 m/s, and then rapidly cooling the melted alloy to form flakeswith different widths and with a thickness ranging from about 0.2millimeter to about 0.5 millimeter. In some embodiments, the columnarcrystals in the flakes may have a width ranging from about 5 microns toabout 25 microns.

In some embodiments, the hydrogen decrepitation process using a hydrogendecrepitation furnace may be those well known in the art, and maycomprise, for example, steps of placing an Nd—Fe—B alloy with freshsurfaces into a stainless steel vessel, filling the vessel with highpurity hydrogen until about one atmospheric pressure after vacuumizing,and then maintaining at the pressure for about 20 minutes to about 30minutes until the alloy decrepitates and the temperature the vesselincreases, this is resulted from the decrepitation of the alloy due tothe formation of a hydride after the alloy absorbs hydrogen, and finallyvacuumizing and dehydrogenating the hydride for about 2 hours to about10 hours under the temperature about 400° C. to about 600° C.

In some embodiments, the mechanical crushing process may be those wellknown in the art, and may comprise, for example, steps of rough crushingin a jaw crusher, followed by mechanical crushing in a fine crusher.

In some embodiments, the jet milling may be those well known in the art,and may comprise steps of accelerating powder particles to a supersonicspeed using an air flow, and then causing the particles to clash witheach other to break up.

(2) The Nd—Fe—B alloy powder and the additive are mixed uniformly usinga mixer to obtain a mixed powder.

In some embodiments, the additive comprising a cobalt ferrite is subjectto a decentralized process prior to the mixing step. The amount of thecobalt ferrite may be about 0.5 wt % to about 10 wt % of the totalweight of the Nd—Fe—B alloy powder. The cobalt ferrite may have anaverage particle diameter of about 10 nanometers to about 150nanometers, particularly about 20 nanometers to about 60 nanometers.

In some embodiments, the alloy and the additive may be mixed in thepresence of an antioxidant, or in the presence of an antioxidant and alubricant. In some embodiments, based on the weight of the Nd—Fe—Balloy, the amount of the antioxidant may be about 0.1 wt % to about 5 wt%, and the amount of the lubricant may be about 0 wt % to about 5 wt %.There is no particular limitation to the antioxidant. For example, theantioxidant may be at least one selected from the group consisting of:polyethylene oxide alkyl ether, polyethylene oxide monofatty ester andpolyethylene oxide alkenyl ether. Particularly, the antioxidant may bean antioxidant commercially available from the Shenzhen DeepoceanChemical Industry Co. Ltd, P.R.C. In some embodiments, the lubricant maybe one or more selected from gasoline, oleic acid, stearic acid,polyhydric alcohol, polyethylene glycol, sorbitan, and stearin.

The mixing process may be those well known in the art. For example, themixing process may be carried out in a mixer.

(3) The mixed powder obtained may be oriented and pressed in a magneticfield to form a parison.

Pressing the mixed powder in a magnetic filed to form a parison may beachieved by using a well known process and a magneticallyorienting-forming-pressing machine. In some embodiments, the orientingmagnetic field has an intensity of about 1.2 Tesla (“T”) to about 3.0 T,and the pressing may be carried out under a pressure of about 10megapascals (“MPa”) to about 200 MPa for about 10 seconds to about 60seconds. The orientation degree of the magnetic powder may be improvedby further increasing the magnetic field intensity. In some embodiments,the formation of the parison is performed in a completely closed glovebox with isolating the magnetic powder from the air, thus avoiding thefire risk due to the oxidation and heat generation of the magnet andreducing the content of oxygen in the final magnet.

(4) The parison is sintered and tempered under protection of vacuum oran inert gas to obtain the Nd—Fe—B permanent magnetic material.

In some embodiments, the sintering and tempering process may be a wellknown process, and may be carried out under protection of vacuum or aninert gas. The inert gas may be any gas which may not participate in thereaction and may be one or more selected from nitrogen, helium, argon,neon, krypton and xenon. In some embodiments, the parison may besintered at a temperature of about 1030° C. to about 1120° C. for aperiod of about 2 hours to about 8 hours, then tempered in a firsttempering step at a temperature of about 800° C. to about 920° C. for aperiod of about 1 hour to about 3 hours, and finally tempered in asecond tempering step at a temperature of about 500° C. to about 650° C.for a period of about 2 hours to about 4 hours. The second temperingstep may further improve the coercive force. Because the cobalt ferritehas a melting point above 1120° C., when being sintered at the abovetemperature, the cobalt ferrite may not be decomposed and melted.

The present disclosure will be described in detail with reference to thefollowing examples.

Example 1

(1) An Nd—Fe—B alloy represented by the formula(PrNd)_(10.61)Dy_(3.5)Tb_(1.3)Fe_(77.55)B_(5.87)Co_(1.68)Al_(0.5)Cu_(0.16)Ga_(0.13)(a %) was prepared by a strip casting flaking process with a rotatinglinear velocity of a copper roller surface of about 1.5 meters persecond. The flake had a thickness of about 0.3 millimeter.

(2) The alloy was crushed by a hydrogen decrepitation process in ahydrogen decrepitation furnace. After absorbing hydrogen to saturationat room temperature and being dehydrogenated at about 550° C. for about6 hours to prepare a crushed powder, the crushed powder was milled viajet milling under a nitrogen atmosphere to produce a powder with anaverage particle diameter of about 3.5 microns.

(3) CoFe₂O₄ with an average particle diameter of about 50 nanometers andan antioxidant (commercially available from the Shenzhen DeepoceanChemical Industry Co. Ltd, P.R.C.) were added to the Nd—Fe—B alloypowder. Based on the weight of the Nd—Fe—B alloy powder, the amount ofthe CoFe₂O₄ was about 1 wt % and the amount of the antioxidant was about0.5 wt %.

(4) The mixed powder was pressed by using a magneticallyorienting-forming-pressing machine in a closed glove box filled with anitrogen gas to form a parison. The intensity of the orienting magneticfield was about 1.6 T, the pressure was about 100 MPa, and the pressingtime was about 30 seconds.

(5) The compacted parison was sintered in a vacuum sintering furnaceunder a degree of vacuum of 2×10⁻² Pa at a temperature of about 1080° C.for about 3 hours, then tempered at about 850° C. for about 2 hours, andfinally tempered at about 550° C. for about 3 hours to prepare anNd—Fe—B permanent magnetic material labeled as T1.

Comparative Example 1

In the process of the COMPARATIVE EXAMPLE 1, no nano-sized cobaltferrite CoFe₂O₄ was added, and the other steps were substantiallysimilar to those of EXAMPLE 1.

The Nd—Fe—B permanent magnetic material obtained was labeled as CT1.

Example 2

The process of this example was substantially similar to that of EXAMPLE1, except that Co₂Fe₁O₄ was used as the additive in stead of CoFe₂O₄,and the amount of Co₂Fe₂O₄ was about 5 wt % of the Nd—Fe—B alloy powder.

The Nd—Fe—B permanent magnetic material obtained was labeled as T2.

Example 3

The process of this example was substantially similar to that of EXAMPLE1, except that the average particle diameter of the CoFe₂O₄ was about100 nanometers.

The Nd—Fe—B permanent magnetic material obtained was labeled as T3.

Example 4

The process of this example was substantially similar to that of EXAMPLE1, except that the amount of the CoFe₂O₄ was about 10 wt % of theNd—Fe—B alloy.

The Nd—Fe—B permanent magnetic material obtained was labeled as T4.

Comparative Example 2

The process of this example was substantially similar to that of EXAMPLE1, except that Co was used as the additive instead of CoFe₂O₄, and anaverage particle diameter of the Co was about 50 nanometers.

The Nd—Fe—B permanent magnetic material obtained was labeled as CT2.

TEST

1. Corrosion Resistance Property

Cylindrical samples with a diameter of 10 millimeters and a length of 7millimeters were prepared from the permanent magnetic materials TI-T4,CT1 and CT2, and then tested on a HAS-70CP type Highly AcceleratedStress Tester commercially available from Terchy EnvironmentalTechnology Ltd, with a temperature of 130° C., a humidity of 95%, asteam pressure of 2.7 bar, and a period of 10 days. The mass loss(W_(loss)) of the permanent magnetic materials T1-T4, CT1 and CT2 wererecorded in Table 1.

2. Maximum Operating Temperature

Cylindrical samples with a diameter of 10 millimeters and a length of 7millimeters were prepared from the permanent magnetic materials Ti-T4,CT1 and CT2, and then heated using a curve measurement system NIM200C(National Institute of Metrology, P.R.C.) from a temperature of 60° C.with a 2° C. increment each time. When the line started to bend at acertain temperature, the permanent magnetic materials reached themaximum operating temperature.

Test results were shown in Table 1.

TABLE 1 No. W_(loss) (mg/cm²) Inflection Temperature (° C.) T1 2.1 190T2 1.8 186 T3 2.5 186 T4 1.5 188 CT1 8.2 160 CT2 2.7 170

It can be seen from the results of Table I that T1 had a W_(loss) of 2.1mg/cm² and an inflection temperature 190° C. and CT2 had a W_(loss) of2.7 mg/cm² and an inflection temperature 170° C., so that the permanentmagnetic material according to the embodiments of the present disclosureexhibited a better corrosion resistance and higher temperatureresistance properties.

Although explanatory embodiments have been shown and described, it wouldbe appreciated by those skilled in the art that changes, alternatives,and modifications all falling into the scope of the claims and theirequivalents can be made in the embodiments without departing from spiritand principles of the disclosure.

1. A permanent magnetic material, comprising: an Nd—Fe—B alloy; and an additive including at least a cobalt ferrite.
 2. The permanent magnetic material of claim 1, wherein the cobalt ferrite is about 0.5 wt % to about 10 wt % of the Nd—Fe—B alloy.
 3. The permanent magnetic material of claim 1, wherein an average particle diameter of the cobalt ferrite is in a range of about 10 nanometers to about 150 nanometers.
 4. The permanent magnetic material of claim 1, wherein the cobalt ferrite is represented by a general formula of Co_(n)Fe_(3−n)O₄, where n is in a range of about 0.1<n<2.0.
 5. The permanent magnetic material of claim 1, wherein the Nd—Fe—B alloy is represented by a general formula of Nd_(a)Re_(b)Fe_((100-a-b-c-d))B_(c)M_(d), where: Re is at least one element selected from a group consisting of Pr, Dy, Tb, Ho, Gd, La, Ce and Y; M is at least one element selected from a group consisting of Co, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d are atomic weight ratios, in which a is in a range of about 1≦a≦10, b is in a range of about 5≦b≦12, c is in a range of about 5≦c≦8, and d is in a range of about 0≦d≦15.
 6. A method for preparing a permanent magnetic material, comprising steps of: mixing an Nd—Fe—B alloy and an additive including at least a cobalt ferrite to obtain a mixture; magnetically orienting and pressing the mixture in a magnetic filed; and sintering and tempering the mixture under the protection of vacuum or an inert gas.
 7. The method of claim 6, wherein the cobalt ferrite is about 0.5 wt % to about 10 wt % of the Nd—FeB alloy.
 8. The method of claim 6, wherein the cobalt ferrite is represented by a general formula of Co_(n)Fe_(3−n)O₄, where n is in a range of about 0.1≦n≦2.0.
 9. The method of claim 6, wherein the mixing step comprises mixing the Nd—Fe—B alloy and the additive in the presence of an antioxidant, and the amount of the antioxidant is about 0.1 wt % to about 5 wt % based on the weight of the Nd—Fe—B alloy.
 10. The method of claim 6, wherein the Nd—Fe—B alloy is represented by a general formula of Nd_(a)Re_(b)Fe_((100-a-b-c-d))B_(c)M_(d), where: Re is at least one element selected from a group consisting of Pr, Dy, Tb, Ho, Gd, La, Ce and Y; M is at least one element selected from a group consisting of Co, Al, Cu, Zr, Ga, Nb and Mo; and a, b, c, and d are atomic weight ratios, in which a is in a range of about 1≦a≦10, b is in a range of about 5≦b≦12, c is in a range of about 5≦c≦8, and d is in a range of about 0≦d≦15.
 11. The method of claim 6, wherein an average particle diameter of the cobalt ferrite is in a range of about 10 nanometers to about 150 nanometers.
 12. The method of claim 6, wherein the magnetically orienting and pressing is performed under a magnetic filed intensity of about 1.2 T to about 3.0 T and a pressure of about 10 MPa to about 200 MPa for a period of about 10 seconds to about 60 seconds; the sintering is performed at a temperature of about 1030° C. to about 1120° C. for a period of about 2 hours to about 8 hours; and wherein the tempering steps comprising a first tempering step and a second tempering step, in which the first tempering step is performed at a temperature of about 800° C. to about 920° C. for a period of about 1 hour to about 3 hours, and the second tempering step is performed at a temperature of about 500° C. to about 650° C. for a period of about 2 hours to about 4 hours.
 13. The method of claim 6, wherein the mixing step comprises mixing the Nd—Fe—B alloy and the additive in the presence of an antioxidant and a lubricant in which based on the weight on the Nd—Fe—B alloy, the amount of the antioxidant is about 0.1 wt % to about 5 wt %, and the amount of the lubricant is about 0 wt % to about 5 wt %.
 14. The method of claim 6, wherein an average particle diameter of the Nd—Fe—B alloy is in a range of about 2 microns to about 5 microns. 